A light glued to the hull isn't nessecary if the hull already has any sort of child subobject (turrets, radar, debris, lods etc). PCS works that bit out on it's own.
The boolean union is the Object Union tool - usually I would avoid it, but it's really the only thing you can use in truespace to accomplish this particular task.
Not sure which version of TS you use, but in mine the Boolean tools are in the middle left of the bottom main control pannel - their icons involve green and red spheres. To use, simply select the first barrel's geometry
, click the 'Object Union' button, and then click the second barrel's geometry
There are two types of actions that can occur with a drag and drop operation in the heirarchy window:
1) Dragging and dropping an existing object group
onto another object or object group will make the first group a child
of the second one, meaning it is on the heirarchy level below it's parent.
2) Dragging and droping a single object (ie, a single light or geometry) onto another object or object group will make the first object a sibling
of the second, meaning it is on the same heirarchy level as the first.
To glue turrets to the model, you will be performing type 1 - since the turrets must be children of the main hull. In this case you'd drag the whole turret object (ie, the highest level of the turrets heirarchy) over the main hull geometry/object group.
Another handy thing I've learned is that when you've finished with your heirarchy and are ready to convert, select something other than your ship and then re-select your ship on it's highest heirarchy level. I don't have a clue what this does, but I recall it being the difference between a PCS crash and successful conversion with a really old model, so I've done it ever since, just in case.