Editing Other Peoples Routes/Courses/Maps

Hello all.
Not sure if this is possible but here goes.

With the abundance of routing and mapping websites and Apps out there. Is it possible to get a route, say from Strava for example.
Import/upload the route into a piece of software or website.
Alter the route so it starts from where i am and joins the other persons route at a point of my choosing and then alter the end of the route so i end up at home rather than at someone else's house.

Upload the route/Course into my Garmin 705 so i can go out and follow it.

You see i'm fairly new to my area and i find myself using the same trails.

I'd also like to be able to do this with routes/Courses i can download from websites like Bikely, any others people can suggest etc.

Any help much appreciated.

MagicDoesMons

P.S. i've tried Garmin base camp but the maps aren't very good unless you've paid for Topo maps which i'd like to avoid!!!