Here's a tip:
If you are dealing with multiple taxiways signs, then it's best to model the signs all at once in a 3D modeling application like Gmax.
Use the map view in FS and take a screenshot of it. Then save the shot as a *.bmp file so that you can use it as a reference in either Gmax or FSDS. You then need the M$ SDKs in order to export the models from Gmax directly into FS9.
However, I "don't" know how to help if you use FSDS.
There is another method to apply taxi signs directly into FS9 without having to use a modeling program, but that method is complicated since you are required to "clearly" understand and know the XML code and its inner workings.
This is your answer to building what you want, Kat's right your asking a big request which require much time on your hand to understand. lol
"Also, is there a way to add custon runway textures to FS??
Is there a way to add custon object .mdl files in the scenery."
Your making more things complicated then you have too, You will become modeler, or designer utilizing ethier gmax or FSDS, these programs allow you do desing whatever you want in 3d form like "taxiway signs". You will have to downlaod plugins from microsoft downloads section, which will allow you to import your model into the sim, with textures animation etc. etc.

Other option, "creatsign.zip" "avsim.com", this program allows you to add custom "airport taxi signs".
