I have four DWL-7100AP's scattered about doing various things. The DWL-7100AP is very similiar to your DWL-2100AP, except that it does both
802.11a and 802.11g/b. They are solid once running, but, well, I know very well how tempermental they can be to get running in "other than usual AP" configurations.
According to DLink, the DWL-2100AP can't bridge off a DWL-900AP:
Likewise, I bought my first DWL-7100AP to do something funky with a router -- only to find out after the fact that it wouldn't work. I was pissed and thought about returning it, but ended up just buying a second one to achieve what I wanted, by just hanging one of them off the router. They worked so well together (SNMP is cool) that I since bought two more of them.
Your best bet, I hate to say, is probably to pick up a second DWL-2100AP to replace the DWL-900AP. The DWL-900AP is old 802.11b, anyway. The two DWL-2100AP's working together will give you 802.11g, and perhaps more importantly, WPA-PSK.
DLink's tech support is an absolute joke. Basically, just consider there to be no tech support except for their web site. I know I'm much more intimate with the DWL-7100AP than anyone at DLink, through trial and error and general playing around. I.e, certain configurations with the DWL-7100AP (such as "repeater" and "client") only work with certain firmwares. On one of my laptops, I have directories devoted to certain configurations for DWL-7100AP that includes the specific firmware that works and the config file for specific configurations. Its easy now to change them around by flashing the firm and the config, but it took a lot of work on my part to piece it together. DLink was of zero help. Your DWL-2100AP should be more straightforward since it only has one radio though.
Basically, your DWL-2100AP is capable of doing a lot of neat stuff -- but don't expect any guidance from DLink. You have to play.