German OOB is full of vehicles and arms captured in first years of Blitzkrieg in Europe. They have even their own system of designation for them. There are some photos of arms what Soviet captured during invasion of Poland in 1939 and how thy use this arms during war
1. Soviet Solders checking polish captured weapons in Grodno 1939
wz.30 HMG, wz.08 HMG and bofors 40mm clearly visible
2. 37mm Bofors wz.36 AT-Gun captured by RKKA
3. Probably also Grodno with 7.92mm wz.28 Browning LMG and wz.98 Rifle
4. Other photo with captured Bofors 40mm in Grodno
5. And with hundreds of Polish 7.92mm wz.98 Rifles/ wz.29 Carbines in Tarnopol 25 of September 1939
6. Destroyed PWS-26 guarded by RKKA army solider 1939
7. Captured Karaś
8. Łoś under new management with red star
Soviets also captured 28 various Polish armour equipment
http://www.magnum-x.pl/czasopismasec/ntw/670
Around 3 7TP tanks and some C7P Tractors which were repaired and used for trials in 1940 together with deep technical analysis of used constructions solutions. One C7P artillery tractor was found recently in Russia acting as a mock-up T-26 in Museum. There are stories that similarly to Germany C7P was used during war in artillery units. 7TP was used in Kubianka proving ground for test and trials and there is no data about using them in units. But the were displayed on exhibitions of captured weapons.
Soviet also captured some number of tankettes TK-3 and TKS. Modern photo of Ex-Polish orginal TKS in Kubianka tank Museum