The German prisoner of war camps, containing millions of Soviet prisoners, were a potential source of manpower. Faced with bad treatment and starvation and a distinct possibility of dying, an increasing number of Russian prisoners volunteered to work for the Germans in exchange for better food and conditions. 
The volunteers were called hiwis, a contraction of the German term for volunteer helper. They were widely used in the Replacement Army and railroad construction units for service duties to free men for the front. On February 6, 1943, the Luftwaffe had 100,000 hiwis in construction and antiaircraft units, replacing Germans.
Hiwis became part of the official table of organization of army units. The infantry division was assigned more than a thousand to perform supply duties, care for horses, and other noncombatant roles. In early 1943 the army replaced Germans with 200,000 hiwis and later an additional 500,000. Other ethnic groups were also used as hiwis. On March 18, 1943, the 715th Division in France used 800 black French prisoners, who volunteered to fill 800 vacancies as wagon drivers, grooms, laborers, and other noncombat positions.
In January 1943 the 9th German Army of Army Group Center included 39,400 Russians, either volunteers or conscripted. The infantry divisions in the 9th Army had a total of 7,700 hiwis assigned plus an additional group of 6,000 attached laborers. When the 9th Army evacuated the Rzhev salient, 21,800 more Russians were seized to prevent their working for the Red Army when it reoccupied the territory, and on March 20, 1943, many were assigned to construction battalions to work on fortifications and roads. The Russians made up one-quarter of the manpower for the 9th German Army. On the Eastern Front in 1943 nearly a million Russians were working or fighting for the German Army. Another 900,000 were employed in Germany to work in factories and on the farms.
The Soviet prisoners were also formed into Ost battalions, equipped with captured Russian weapons, and used to fight the partisans. In early 1943 the Germans had 176 Ost battalions; many formed by anti-Communist ethnic minorities from the Caucus, In May 1943 there were 32 Turkestan battalions, 12 Georgian battalions, 11 Armenian battalions, 8 North Caucasus battalions, 16 Muslim and Azerbaijan battalions, and 10 Volga Tartar battalions. By June 1943 there were 320,000 Ost troops.
Ost battalions also replaced Germans in the occupation divisions in France. On January 27, 1943, the German High Command ordered the German divisions in France to send one of their infantry battalions to Russia and in exchange received an Ost battalion. The Ost battalion had German uniforms, but Russian weapons. The first ten battalions were quickly followed at a rate of three battalions in exchange for a single German battalion.
 Hilfswillige: Auxiliary Volunteers. After the invasion of the USSR, many thousands of Soviet citizens volunteered to fight the Soviet regime. At first, the German government refused to use them, but later relented (no doubt in the face of mounting casualties) and allowed the German Army to use them in non-combat roles. Hilfswillige served as auxiliaries to the front line troops on various support tasks such as construction or carrying ammo.
 Already post June 1941 the army had these Hiwis in their KStN (Unit organizations). The KStN says how many Hiwis are authorized for the unit. Which position they have can be determined by the commander, but they must be in the Tross section.
In this KStN it is the last point of the additional information at the end of the document.
Also note that the "authorized" numbers of Hiwi's reported by units to be an accurate figure of the numbers that were employed. This is especially true during the early years of the war when "official" Hiwi policies were still unwritten. A lot of the field "improvisations" to solve manpower problems were either unreported or downplayed.
This is also true in the case of Hiwi's joining combat formations - unfortunately there are no definitive records of when these laborers became soldiers.
After September 1943 thousands of Italian soldiers in Balkans and elsewhere were incorporated as Hiwis in Wehrmacht as an alternative to deportation in Germany.