C::B can not convert from makefile to C::B build-system.
This has to be done manually.
The difficulty highly depends on the size and complexity of the project.
But you can create a custom makefile project:
- Create an empty project with the wizard,
- add all files you might want to work on,
- go to "Project -> Properties",
- check "This is a custom makefile",
- edit the makefile name if needed,
- change the path to the makefile if needed,
- fix the make commands in "Project -> Build options -> "Make " commands", for the target and/or the whole project.