I want to make spiced pumpkin buttercream for the very first time

"Pumpkin spice" anything is just that: the spices, not the pumpkin.  Same for "apple pie spice"  it's the spices used, not the actual pie part.  So you could either use the creamer, or the spices, but I'd stay away from the real pumpkin  

i've never made it but if i did i can think of several different ways to go --

i'd either sub pumpkin puree (not the pumpkin pie filling) for the liquid in the recipe or i'd cook down the pumpkin puree to remove the water and i'd probably add it as an extra ingredient -- but pumpkin oxidizes a little too so don't know how that darkening effect will affect the Icing --

i think the raw pumpkin might need some added cornstarch 

then i wonder how just regular pumpkin pie filling would do

tons of ways to go and test

just some thoughts for you 

but all that to say i've never used those creamers -- just not for me

best to you

...and i would actually put the pumpkin spice emphasis in a filling or in the cake itself 

Thank you

Pumpkin spice: 1tsp cinnamon, 1/2 tsp nutmeg(or allspice), 1/4 tsp of ginger

or buy the Pumpkin spice seasoning from the store.

I put cinnamon in a frosting last week, it was one of those where you didn't taste it at first, but then bam! you got hit by the cinnamon at the end. I wouldn't put coffee creamer in the cake though if it's the non-dairy kind, made my cupcakes sink when I used non-dairy. In icing, coffee creamer is nice though! :)