Is there a way to import someone else's webhelp project so that I can take over and work on it?

Ok, assuming that you have access to the other person's machine, the RH project files should be in a folder on their c:\ drive (in like c:\projects\[project_name]\ or My Documents\My RoboHelp Projects) - once you've located it, just zip it up into a single .zip file and take it over to your machine. Unzip it - the best practices place is c:\projects\{project_name]\ and start RH up. Then just browse to the location of the project and open the [project_name].xpj file. Now it's your project ;>)

If you don't have access to the RH source files, but only have the WebHelp output files that it produced, you can get a working RH project back out of it by using a process known as "reverse-engineering" - see grainge.org for a free way of doing this or wvanweelden.eu for a paid method.

If either of these ways don't suit, maybe you need to explain your situation a bit more & then we can try helping you.