You can use the same Wilton royal icing method for mums to make mums with regular buttercreme. It would help if the buttercreme was stiff, however. You can use a rose nail and squeeze up a short and round foundation and then do your mum, then use a spatula to slide it off and put it on the cake. Or, you can pipe your mum right on the cake and just keep turning the cake on the turntable as you pipe the petals.
Tip 81 makes the curved petals for the mum.