The trimming part is easy. The preferred method is to use Edit > Solid Element Operations (SEOs). You will want to use "Subtraction with upwards extrusion". You can also use the older "Trim to Roof" command (also in the edit menu). The former maintains a relation between the two elements so that the cut element will follow changes in the roof, the latter inserts the cutting info into the part itself so that it no longer tracks the roof and keeps the cut even if the roof is deleted.
To get the top mullion is a little trickier. You either need a library part that will do this for you, or model it yourself with the roof tool (of course this way you can use the mullion to do the cutting).
