To port maps from Battle Island or History Line, you need the bi2cf utility. Depending on the distribution channel you got Crimson Fields from, you may have to compile it yourself. Otherwise you can skip step 1.

1. compile bi2cf

1a. ./configure --enable-bi2cf
plus any other options you may want to pass

1b. Change into the tools directory

1c. make bi2cf

Then copy the maps from your Battle Island disk (the map folder). Now we got to convert the maps:

2.

for i in *.fin; do bi2cf -f b $(basename $i .fin); \
  cfed BI-$(basename $i .fin).src \
  --tiles PATHTOCRIMSONSRC/tools/default.tiles \
  --units PATHTOCRIMSONSRC/tools/default.units; done

3. Then place the *.lev files in your level folder. Don't expect too much. :-)