Yes you can do that. You can manually edit the panel.cfg of the acft. panel you want to put it into. You really need to know what you are doing here if doing it manually.

To make it simpler, get a program called CFGEdit. It is a free panel design program, fairly simple. Check to use FS2000, browse to your FS9 and there ya go. Look at the panel.cfg of the panel that has the gauge you want to move, note the EXACT name. Now Start CFGEdit and browse to the acft. you want to add that gauge to. Open up the panel, here you might have to remove a gauge to make room for your new one or move things around. Then hit the "add a gauge" button and browse to THAT gauge you like. Position it around to were you like, size it to your liking and then save. CFGEdit makes a backup automatically for you.

CFGEdit will not display XML gauges, but you can still use them, you just won't have a visual of it in the panel program.

Play with it for a bit and I bet you will be modifying your own panels to your liking in no time.   

There is a payware version  that will allow you to work with XML gauges. The program is called FLIGTHPANEL STUDIO it is the big brother of CFGEDIT It does the same thing as cfgedit since it is the same thing the only differance is the ablity to read and make XML gauges.
You can find it at www.flightpanelstudio.com