How to integrate external map->odom transform into rtabmap?

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For your first question, rtabmap will still detect loop closures, but the map frame inside of the map topic won't mean the same thing than external TF map frame. The map may look odd in rviz.

For your second question, the short answer: you can't.

The long answer, it depends what is represented by your external map->odom, thus some other data can be fed to rtabmap instead:

• If your external map->odom is coming from another SLAM system (that is working in its own reference frame), there is not that much you can do, as both SLAMs will drift differently from each other (as they won't detect the same loop closures). You would have to chose one or the other.
• If the external /map -> /odom represents a world frame (e.g., combined GPS + wheel odometry using robot_localization package), you may feed the pose /map -> /base_link of that system as global_pose input topic to rtabmap node, which will use it as prior. The internal graph will optimized in that coordinate frame, thus /map -> /odom published by rtabmap will include your external global pose estimation. The TF from that external node would have to be disabled, and let rtabmap provide /map -> /odom.
• If your external pose is GPS, feed it /gps/fix as input topic of rtabmap.

In localization mode, you can disable /map -> /odom published from rtabmap, and feed its localization_pose to some other node doing fusion with other global poses.