You actually can't add filters to the basic search beyond what is already available, but you can use the advanced search to filter by any non-calculated field in the record. Start by building whatever filters you can, then click the 'Advanced' option at the bottom. You should have some search language already loaded into the top window. Start here by clicking the 'AND' button to add a new criteria. Then, make sure you select the appropriate record type on the lower left list and then find the name of the desired field in the lower right list. Double click on that field and then click 'Value'. Find the value that you want to search for and double click that. Then click 'OK'. You will see the new criteria appear in the search language in the top area. To add another filter, just hit AND again and repeat the process.