As FT said, you can't access the RAM directly.
But is that what you want to do? Or are you looking for extra storage? Two different things.
Networking your laptop to a desktop and sharing the drives is a (relatively) simple thing to do. That would allow you to backup files from the laptop on the desktop, or store things you don't need frequently on the desktop.
I suggest you consider picking up a USB external hard drive, which adds lots of storage capacity, can be had for under $200, and won't require mucking around with network settings.
Or, for much less $$ (but much smaller storage) you could get a memory stick or two.
But if you're after more RAM (say, to increase the performance of the laptop on certain programs) then it has to be installed into the laptop itself.
-Glenn