I have made a  batch of SMBC for a cake this weekend.   It looks like our weather is going to be high 80s-low 90s.    They are having the party possibly outside.

My question is:   Can I add some shortening to the batch that I have already made to give it more stability in the heat?

Thank you.

I assume that your event was today, if so, what happened with your BC?  If it's tomorrow, you may have time to make more BC.  I was going to make SMBC for an outdoor August reception and was told that any BC with butter won't work, even with shortening.  I was told to make a 100% shortening buttercream.  So I will go this route and add butter flavor to it.