Special Flight Setup Question (COF)

Got a question if I may please. I've been fooling around with something and
I think I'm close to getting it right but could use a little advice here.
What I'm trying to do is create a flight without a flight plan from a
specific airport I intend to depart from, fly around a bit, and return
to...but I want to see if this can be done in a special way. What I'd like
to do is set up local weather for IFR conditions at takeoff...say solid
stratus overcast, about 1/2 mile visibility, and moderate rain. I'd make the
tops at about 2500 feet so that the takeoff and climb would be on
instruments, but breaking out in the clear for the local flight. Now this
doesn't pose a particular problem so far, as I've set up this weather ok in
the weather section while creating the flight under user defined weather for
a specific station....great!
First question; for a setup like this, is the weather above the upper limit
for the overcast default clear, or should I be selecting clear in some way
in the weather setup? I'm getting clear which is what I want, but I don't
know if it's an accident or if the sim's weather is defaulted clear over
whatever tops you specify.
Second question, and this is the one I need the help with really.
After flying around awhile in the clear say within 50 miles, I'd like to
file IFR in the air and request a specific approach back at my departure
airport....which should of course still be IFR as I left it.
I've found the ATC select to contact approach control, then the option to
file an IFR flight plan, but this is where I'm having trouble. The plan box
asks for a departure airport. I don't have one since I'm already in flight.
If I don't list a departure airport, the plan doesn't give me any way to
proceed. Can I file to go back to the airport I left and get the ILS for a
specific runway, and if so, how is this accomplished in flight?
Sorry to be so long winded on this, but I'd really like to get this right.
Here's the reason. I tried it without the flight plan filed, used the GPS
for vectors to the OM in the soup, then collided on the ILS with an aircraft
ATC had cleared properly for the approach.
There's GOT to be a way to do this properly.........anyone?????
Thanks much,
Dudley