Presuming you have something akin to the following:
On disk:
- ./solution1.sln
- ./solution2.sln
- ./solutionN.sln
And a single ccnet project:
- Msbuild task -> solution1.sln
- Msbuild task -> solution1.sln
- Msbuild task -> solutionN.sln
What you are asking for is ccnet to react to what is outside of its environment. This isn't possible, however it would be possible to get another tool to do so.
Possible options:
1. Custom Msbuild project
Create a specific msbuild project which finds and invokes msbuild on all solution files it finds. Call msbuild on this project alone. It should be possible to do this with vanilla msbuild, see https://msdn.microsoft.com/en-us/library/z7f65y0d.aspx
2. Batch files
Find all files, and for each file execute msbuild. Ensure the output is logged to an XML file (msbuild switch - I believe) and merge in the result in the publishers section.
See How to do something to each file in a directory with a batch script
3. Single solution
Create a single solution, which contains all the projects from all solutions (1 to N) and call msbuild on this once.
This solution file would be need to be updated each time a new project comes along however.