Well, gMax has an SDK (FS2004 SDKs in my signature) which states that it can create a solid surface to land on, which is independent of the ground.

I believe this was in part due to multiple requests after FS2k2 for something that would allow this type of landing.

There are several scenery downloads out there that utilize non-ground based landing strips, such as the Independence Day City Destroyer scenery, created for FS2k2.  You can get them here at SimV.  Try asking the designers what they used to create the runways.

But as far as I know, you cannot make mobile airstrips, if that is what you meant.  It doesn't seem to be possible in the current FS environment.