Q: How can I transfer photos to my ipad without having to overwrite the ones already on there?

when you sync, the process replaces all the photos on the pad with the ones you have included in the photos tab to be synced. 

If you have photos there that are not on your laptoip, therefore cannot be resynced, simply upload them to i phoho or whatever photo mananger you are using, and include them for resyncing.  It does work pretty well.

If you want to skip the whole sync thing, then

e mail them to your self

or use a cloud like dropbox.  Put them there, then bring them, down opnto the pad.

or

use a file transfer app like photosync.

Another way. You can use a USB flash drive & the camera connection kit.

Plug the USB flash drive (works the same with an SD card) into your computer & create a new folder titled DCIM. Then put your movie/photo files into the folder. The files must have a filename with exactly 8 characters long (no spaces) plus the file extension (i.e., my-movie.mov; DSCN0164.jpg).

Now plug the flash drive into the iPad using the camera connection kit. Open the Photos app, the movie/photo files should appear & you can import. (You can not export using the camera connection kit.)

Using The iPad Camera Connection Kit

http://support.apple.com/kb/HT4101

 Cheers, Tom