In real life, there is a clear distinction between VFR and IFR flying. There can sometimes be a blend - VFR on top, pop up IFR, SVFR etc, but even then, you are clearly within one regime or the other at any given point. It is possible to file IFR in the air. Pop up IFR is sometimes frowned upon especially if it’s clear that you’re just jumping the departure queue waiting for your release.
However, if the weather moves adversely and you’re approaching IMC, an IFR rated pilot would absolutely request airborne IFR assuming the plane was properly-equipped.
Instrument approaches, e.g., ILS, RNAV, etc., are IFR procedures and are not offered to VFR flights. An IFR rated pilot (or a CFII with IFR student) flying VFR in real life can request an extended straight-in pattern entry from the controller, controller’s workload permitting. This “wink and nod” method between pilots and controllers allows controllers to vector VFR traffic into an IFR arrival corridor, providing opportunities for practice approaches. Officially however, the pilot is required to fly VFR with reference to outside, see and avoid, clear of clouds etc.
In MSFS, depending on the plane you’re flying, you need to punch in a full flight plan into the FMS (not simply direct to and select an approach). Then MSFS may recognize that you have something that looks like an IFR flight plan in the “box” and give you the option to request an IFR clearance from the controller.