...if i understand you correctly you have the underline feature in the character palette used and now wish to increase the space between that line and the word it is applied to...
...in short you can't change this in illustrator, it does have typographic controls, but nothing on the level of indesign, which is largely why Adobe don't include as much in illustrator as indesign because they want people to buy indesign/creative suite...
...the only workaround in illustrator is a manual operation and not a dialog controlled feature, as it is in indesign...
Andrew