IFR flight plan filing question

working towards my instrument, only about 7 hours so far, but taking the
written next weekend and feeling pretty confident about everything so far.

question i had planned to ask my instructor, but maybe i can get an answer
today...

suppose i'm flying from denver to rapid city on a VFR flight plan, or no flight
plan at all. i check with FSS about conditions at RAP, and they have changed
to IMC, or forecast to change to below VFR mins at arrival.

could i then file an IFR plan with FSS from the air?, head to the nearest
airway and switch to ATC?

if yes, i could then open with ATC from wherever i'm at? and if i was on a VFR
flight plan, would i just be changing it to a composite flight plan?

when my VFR x-countries i usually fly direct, but i always use flight
following. i've read posts about problems with filing direct- but i'll save it
for another time

thanks in advance

james...