Well... The manual says u can't!! But, there is a strategy for this also! First, hire as many soldiers as the amount of population u want to move... After the soldiers are ready, mobilize them and then move the army to another province where u want to send the population... Now, disband the army! In this way u lose 50% of the resources u've spent to hire the soldiers, but u have moved 100% of the population from 1st province to the 2nd one... I think it's a really useful strategy, what do u think?
Ofc i think the best thing to do is to hire soldiers in a plain province and move the army in a mountain one... To increase resources production in mountains!
P.S. it's not mine strategy, i only know it...
hmmmm its good also it is best if you use light spears for transferring and also make sure you have spare resources...