You need to add your the configuration information for the membership provider to the web.config file for Central Administration. This will allow Central Admin to find from both AD and your own membership. There are examples on Technet for a SQL Server and an LDAP membership provider. Note how it mentions that the membership provider info needs to be added to both the web app's web.config and to Central Admin's. If you're using a Role Manager though, don't put that info in the Central Admin web.config, or at least not unless you really know what you're doing and why. :)
Once you do this the people picker should be able to find people from your FBA solution's membership provider. One little caveat if you happen to be using the same AD store (or some kind of replica) for both Windows auth and LDAP auth. You'll need to use the "prefix" of your membership provider name when specifying the user, otherwise the people picker will always lookup the native AD version of that user. For example, looking up "jdoe" will always find "domain\jdoe" first. If you want jdoe from an LDAP provider pointing at that AD, you'll have to explicitly put in "ldap:jdoe" (substituting whatever you called your membership provider in the web.config for "ldap"