Is there a way to place a curve between two previously defined landmarks?

Yes, you can place a curve between two existing landmark points by following these steps:

1. Select the Curve primitive from the menu Edit | Add Curve.
2. Place the start and endpoint of your curve near the two landmarks you want to join with.
3. Select the Hand tool from the menu Edit | Edit Landmarks.
4. Hit the space bar to clear the selection (if any).
5. Click on one of your originially placed landmark points. This should color the landmark red. (It will be colored yellow while you hover over it.)
6. Click on the nearest endpoint of your curve. This should color the endpoint white. (It will be colored yellow while you hover over it.)
7. Join the two point by selecting Edit | Combine Landmarks. This will join the curve endpoint to the pre-existing landmark point. The joined points will take the position of the red-colored point.
8. Repeat this process for the other curve endpoint.

David, 

Thank you for your reply. We are using the method you have suggested.

 A few issues: When working to join curves to landmarks, if we don't like the result, UNDO causes the software to become non-responsive.  We have shut down and gone back to our models and find that landmarks and the index list are no longer the same (38 in the index shows ecmr, but on the model the landmark is labeled 38 ecml, and 39 is gone). Sometimes curves have cm for units instead of mm. If you have any further suggestions, we would be grateful. Thank you,