You can convert the files simply by using Filemaker 11 to open them. Filemaker makes new converted copies leaving the originals untouched. The resulting files will likely need additional post conversion work before they are fully functional.
Here's a Knowledge Base article on the Subject
I know when I made my jump from FM5 to FM9 I had a few conversion problems. I was told to try to convert my FM5 to FM7 because that is when the extension change occured, then take my FM7 to FM9. I think it is the same for FM11 but at the time I did not have FM7 so I had to just deal with the conversion issues.