You have to use another entity (most likely one with just a local interface) to model this type of relationship. The new TeamPlayerEJB would be a member of two one-to-many relationships.
You many run into problems with the database design being outside your control. JBoss 3.0 currently doesn't support using relationship foreign key fields for primary keys, so your relation table must have an additional cmp field for your pk, which is not probably the design you will get from you db people.
This on my list of features for 3.0 final, but it is not at the top of the list. When, I do implement it you will be able to add new cmp fields, say PlayerFk and TeamFk, that you will map to the relationship fields. Then you would just create a compound pk for these fields.