I don't think you can do that. The best option will be to either draw the weld symbol using the sketch entities as you'll do for the rest part of the block or save the weld symbol in the design library for quick insertion.
I agree with Deepak. Draw the symbol and save it in the design library.
One thing I will add is you can add attributes to the block for the text
if you think it is necessary to change the text within the block. This would
enable you to change it without having to explode it.