Can active flight plan be sent for the aircraft in the air?
Your question is short on details, but in general, yes this is possible. A flight could be coming from anywhere, for example:


Another jurisdiction (FIR)
Uncontrolled airspace
A beach somewhere


In these sorts of cases, you may see the first FPL message after the aircraft has departed. The departure aerodrome, if known, may appear in the ADEP field (field 13a), or it may be ZZZZ for an unknown aerodrome, or it may be AFIL for an "air filed" flight plan.