Query table and then display in postbit

[Gearloose]

I have a custom table with some fields like this:
sometable(someid, somethreadid, someuserid, somefield1, somefield2)

How can I display in postbit (e.g. under posts#) the value in somefield1 and somefield2, based on threadid and userid?

I guess I have to use a variable (let's say $myvalue) in postbit template which takes a value by a query like:

PHP Code:

        $myvalue = $DB_site->query_first("
                SELECT somefield1, somefield2
                FROM " . TABLE_PREFIX . "sometable
                WHERE (somethreadid = " . intval($threadid) . ") 
                AND (someuserid = " . intval($userid) . ") 
        "); 


where: $threadid -> the thread I view
and: $userid -> the userid of each user's post in this thread
(The pair somethreadid/someuserid is unique in the table.)

Then I guess I have to use $myvalue[somefield1] and $myvalue[somefield2] in the template.

Is this correct?
How can I accomplish this?
Should I make a new php file or add this query in an existing one?
What else should I add in the php file besides this query?
Do I have to make a new teplate, or just add this variable in postbit?
Any help?