I did something similar which was activated by a link in their profile accessible to only the SuperAdmin. For this to work I made sure all my private forums where assigned to one Category. I aslo had the requirement to set the user as a moderator for each private forum at the same time - you can strip that part out from below
<!--#INCLUDE FILE="config.asp" -->
<!--#INCLUDE FILE="inc_func_common.asp" -->
<%
Dim EMP
EMP = ChkString(Request.QueryString("recid"),"SQLString")
Dim strSql, conn
Set conn = Server.CreateObject("ADODB.Connection")
conn.Open strConnString
strSql = " SELECT FORUM_ID INTO FORUM_TEMP2 FROM FORUM_FORUM WHERE F_PRIVATEFORUMS=6 "
conn.Execute strSql
strSql = " SELECT FORUM_ID INTO FORUM_TEMP3 FROM FORUM_FORUM WHERE F_PRIVATEFORUMS=6 AND CAT_ID=2 "
conn.Execute strSql
strSql = " ALTER TABLE FORUM_TEMP2 ADD MEMBER_ID INT"
conn.Execute strSql
strSql = " ALTER TABLE FORUM_TEMP3 ADD MEMBER_ID INT"
conn.Execute strSql
strSql = " UPDATE FORUM_TEMP2 SET MEMBER_ID = "&EMP&" "
conn.Execute strSql
strSql = " UPDATE FORUM_TEMP3 SET MEMBER_ID = "&EMP&" "
conn.Execute strSql
strSql = " INSERT INTO FORUM_ALLOWED_MEMBERS (MEMBER_ID, FORUM_ID) SELECT MEMBER_ID, FORUM_ID FROM FORUM_TEMP2 "
conn.Execute strSql
strSql = " INSERT INTO FORUM_MODERATOR (FORUM_ID, MEMBER_ID, MOD_TYPE) SELECT FORUM_ID, "&EMP&", 0 FROM FORUM_TEMP3 "
conn.Execute strSql
strSql = " DROP TABLE FORUM_TEMP2 "
conn.Execute strSql
strSql = " DROP TABLE FORUM_TEMP3 "
conn.Execute strSql
Response.Write "<table align=""center"" colspan=""0"" border=""0"">" & vbNewline & _
"<tr><td align=""center""><br /><br /><p><font face=""" & strDefaultFontFace & """ size=""" & strDefaultFontSize & """>Access privileges have been granted to the nominated Employee." & "</p>" & vbNewLine & _
"<p><a href=""JavaScript:onClick=window.close()"">Close Window</a></font></p></td></tr></table>" & vbNewLine
Response.End
conn.Close
%>
Edit: Actually, looking at what I did again, the code first gives the user access to all 'Allowed Member Only Hidden' forums then sets them as a Moderator for all 'Allowed Member Only Hidden' forums with a Category ID of 2.