Adding X++ code on runtime.

I want to add a X++ code when a specific function occurs

(Such as SalesLine::insert).

Before/after the insert.

Check compilation of code, etc...

Can I do that?

I see once an ability in AX to write external X++ code (Something about template - I don't remember exactly ...)

I know that a whole code is available, but is there ability to write partial code (that should be put in a code such as salesLine::insert()).

For that ability should the client have license for developing on his environment?

Thanks :)