Hi Rudl, here's the basic extension process.

I'll focus on the connection between these 2 pieces here:



First select the inside faces and use Edit > Separate so you have just these 2 pieces here:



Now you want to get 2 bigger planes for each of these so you can intersect the 2 extended planes to find the common juncture curve. The easiest way to do that is to select all the edges on both of those surfaces (select one edge on each and then do Ctrl+A for select all is the quickest), and then push Delete to do an "untrim" operation where the current trim boundaries will be removed and the full "underlying surface" that's underneath all the trim curves will be exposed. This underlying surface is already a large extended plane so you won't need to do any additional extension. After you have done the untrim (again, that's by selecting all edges and doing Delete), you will have this kind of result with 2 big planes:



Select both planes and run Construct > Curve > Isect to generate the intersection line between them:



Now it's probably easiest to select that intersection curve and do Ctrl+C to copy it, then undo things back until you have your previous trimmed surfaces again and then do Ctrl+V to paste the intersection curve back in. That will give you this result:



That curve there is the intersection between the 2 extended planes, that is where you probably want the interior boundary to be at. You can either build some little fragments to fill in that area or you can extend the current edges to make a new big outline and then remake one big planar surface that has this new intersection line as its boundary rather than the previous unextended one.

You'll need to do this between all surfaces and in areas where multiple planes are coming together you'll need to trim these intersections with one another to make the final boundaries.

Hope this is going in the right direction...

- Michael