OTL immediately post-WW2 a few airframes were converted from piston to jet power.
The Russians simply hung (Junkers Jumo 004) jet engines under the noses of existing Yak 3 tail-dragging fighters. Yak 15 was unique in being a jet-powered tail-dragger.
Sweden re-engined a batch of SAAB 21 twin-boom ground attack fighters with British deHavilland Goblin jet engines.
The record belongs to the Fairchild C-123 which flew as a: glider, piston-pounder, Jet, turboprop and finally a combination of pistons and jets! C-123 was initially designed as an assault glider and flew the bulk of its missions as a twin-engined, piston-pounding transport (R2800). Vietnam War-vintage C-123 versions had a pair of J85 booster jets hung under the wings to improve take-off performance. At least one C-123 flew as a pure jet with a total of 4 jet engines hung under the wings. The twin-prodded Jet engines were borrowed from B-47 bombers. A turboprop prototype was even built for a Royal Thai Air Force contract, but never entered production!
WI more WW2 airplanes were retrofitted with jet engines via simple bolt-on conversions?
Rumour has it that the Westland Whirlwind twin was contemplated for quick conversion to jet engines......