It is possible to recording via the built in mic while playing a pattern ?

thanks

Nope.

I'd suggest buying a cheap metronome (some tuners have them built-in). You can get a decent pocket electronic metronome/bpm counter for under $25.

When you record in use the metronome to keep time for whatever you're rapping/playing.

Actually I've used Reason's metronome tons of times because I liked the sp-404's workflow so well.