I have two weldment parts in an assembly, how can I add a new structural member (part) AND apply trim/extend with respect to the other structural members in the assembly?

I just asked the weldments expert in my office and says it is by design that you can´t perform weldments features within the asm context. Although we dont know why.

In general terms, assembly is to assemble components (like fasteners being used to assemble two plates) whereas weldment is when you weld them. 

So create you components in single part and build configuration to show them as pcs. Weldment are not meant for assembly.

Weld functionality as it is used is defined around how is it made.

If I have 2 weldments mated together in an assembly it is by design intent that these 2 will be manufactured separately from each other. The trim extend function is based upon this manufacturing principle. All things contained in a weldment are onto it's own function. Hence outside direct trim is not an available option.

As far as software goes could this be an option to use? Certainly, I would not mind being able to utilize more convenient things like this if I needed them.

Having designed and welded myself more structures than I care to remember, I think the way the software sets this up is to make you think of "how" your parts are going to be made.

In the case of what you are describing for your parts I would say you really only need 1 weldment and not an assembly anyways. You can change materials/ appearances per body per occasion.

As far as separate parts you can detail individual bodies and pull your weldment apart as you deem needed.

Just my 2 pennies worth.