Yes,you can :smileyhappy:
it's all depeands on the script of the HTML your are building.
just make the username field as hidden field in your HTML script..(and put the desire username that allowed multiple login + assigen to this username password)
here is a good example: (you should create a username called voucher in your internaldb)
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<!DOCTYPE HTML PUBLIC"-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
</head>
<body>
<form name="form1" method="post" action="/auth/index.html/u">
<input type="hidden" id="user" name="user" type="text" value="voucher" accesskey="u" />
<input type="hidden" name="cmd" value="authenticate" />
<input type="password" id="password" name="password" />
<input type="submit" name="Login" value="Login" />
</form>
</body>
</html>
Also,do some reading over here:
http://community.arubanetworks.com/t5/Unified-Wired-Wireless-Access/Single-Click-Captive-Portal/td-p/66478
https://arubanetworkskb.secure.force.com/pkb/articles/HowTo/R-154