I'm sorry, I don't know what a "striker" is.
Is this what you're calling a "striker"?
In Winnipeg, where I live, most people call this tool a "jointer". (But, lots of people call it a "spoon", too.) I guess it doesn't matter what you call it, as long as the other person knows what you mean.
I wouldn't use a jointer to pack the mortar in. You use a jointer to compact the surface of the mortar to make it more water resistant. If you put the mortar in with a jointer, how are you going to then compact the surface of the mortar to make it more water resistant?