Did the Germans ever use captured Allied aircraft against the Allies?
Not sure about the Germans, but the Italians used a P-38 (from the site 12 O'Clock High):

The piece about the P-38 captured by the Regia Aeronautica is
extremely correct (at last, I was sick of reading accounts or
questions mutuated from the complete fantasy of Martin Caidin...! ),
except for one detail: the plane shot down on August 11, 1943 by Col.
Angelo Tondi with the P-38 wasn't a B-24 but a B-17 instead...

and later on the same post:

This is extracted from the diary of 2nd Lt. James Franklin Boston:
&quot;August 11, 1943-Mission to Terni, Italy 06:00. Carried 6000 lbs.
demos. (demolition bombs). Flak moderate. Coming back 60 miles from
Italy intercepted by ME-109 and captured P-38 with enemy flier flying
it. Nobody fired at first, thinking it friendly. Saw it slip in a
formation from behind and shoot down a B-17. Everybody fired at it
then including myself but my gun jammed. 20mm shell came by my head
while I was looking through astrodome and went into No. 2 engine. P-38
was not shot down.&quot;