Do you happen to have a picture of the layout? I too wish for a keyboard with more keys, especially vertically. But besides an extra row or two of F-keys, I haven't come up with any logical extensions.
Note the blanks in the lower right there do have switches under them so you could put keys there as well (that's where mine has Hold and UnHold incidentally). One warning is that the blank keycaps and relegendable keycaps (the ones that are a clear plastic cover you can stick a bit of paper under) are mostly sold in bulk to system integrators and it's expensive to get them in small quantities. So if you're in the market, you probably want one with as many of those as possible. PrehKeyTec's software supports printing legends for them but it's pretty basic so you might want to use something else to design them.
Note the blanks in the lower right there do have switches under them so you could put keys there as well (that's where mine has Hold and UnHold incidentally). One warning is that the blank keycaps and relegendable keycaps (the ones that are a clear plastic cover you can stick a bit of paper under) are mostly sold in bulk to system integrators and it's expensive to get them in small quantities. So if you're in the market, you probably want one with as many of those as possible. PrehKeyTec's software supports printing legends for them but it's pretty basic so you might want to use something else to design them.