Yes you can, if it starts out as non restricted and the finished product still meets the criteria for non restricted you can do whatever you want to it.
I think you may be thinking of the rules for restricted rifles. Can't shorten the barrel of a restricted rifle even though it should technically make no difference since it would still be restricted. Canadian firearms law makes a lot of sense eh? I know a guy that takes his 22 inch hunting rifles to a smith and turn them into 18 inch, this is no different and only a semi auto has to remain over 18.6.