Hello.
Is it possible to have substitutions that are bold?
All kinds of creative but unhelpful responses to that question spring to mind. 
But to keep it basic, no, I don’t think so. Substitutions (meaning Tools > Options, Corrections tab > Edit Substitutions, right?) only replace a text string with another text string; they don’t include formatting.
I suppose, if you knew the RTF code for begin bold and end bold, you could include them in the substituted text string and see if it worked.