conditional flatten mapping

I have the following flatten mapping in my ant file.

      <path id="build.classpath" path="${deplibs}"/>
      <pathconvert property="mf.classpath" refid="build.classpath" pathsep=" ">
        <flattenmapper/>
      </pathconvert>

However for a couple of the paths in the deplibs variable I want to
make these full instead of relative paths. Is there anyway to do a
conditional in the pathconvet in ant to say make these flat but do not
flatten these paths?