I think it's possible. You will just have to create a link between your cus:recipients and your cus:services tables.
For drop-down fields: they are displayed on visual query editor when the field has the enumeration property set. So, in order to do that, you will have insert the following lines into your cus:services schema:
<enumeration basetype="string" name="forumInput">
<value label="Label of your 1st value" name="nameOfYourFirstValue" value="0"/>
<value label="Label of your 2nd value" name="nameOfYourSecondValue" value="1"/>
<!-- You can create more lines inside this enumeration -->
Then, on your attribute label, you can add the enum field just as the following: