What exactly are you trying to do?
1. move a soldier from "unmodded xcom" to "unmodded openxcom" ? should be trivial
2. move a soldier from "unmodded xcom" to "modded openxcom" ? should also be trivial, but if not, use the method SupSuper described
3. move a soldier from "modded xcom" to "unmodded openxcom" ? if there are any changes in modded xcom save format, we won't be able to import it
4. move a soldier from "modded xcom" to "modded openxcom" ? same as point 3
5. move a soldier from "unmodded openxcom" to "modded openxcom" ? should be trivial