Its ability can only go half ways.
In the first screen, for writing your nickname down, you could make that one into php and auto include the username they are using in the boards. That is as far as it can go. No big deal.
JPilot uses irc servers, so if my nick here is "MTO" and it auto-fills that nick in for me, that is no guarantee I will be able to use the nick when I log into the irc server. You need to have your nickname registered through the irc server, no way any php script will do that for you, except maybe if you host your own irc server, that I really doubt. I use jpilot, and can log in with MTO, but because I have it registered. That is, under the "irc-hispano.org" network. I wouldnt be able to use it elsewhere.
So that´s it... all you can do is just grab the nickname you use on the boards, and fill it in for the first jpilot screen. Safest thing to do is fill it in with an extension, so that since most probably "MTO" will already be registered, you would autofill it in with "MTO_ubb" or something like that. I doubt anyone registered that name. This basically avoids people entering as, for example "martha", only to discover it is already registered in the irc network, and see their name automatically changed to something like "inv93274".