Pushing bars to an existing model

#### Description:
I have an existing model with many members, surfaces etc. The node numbers, line numbers, member numbers etc are not all in one series (e.g. 1-100 exist and then also 250-345).

When I push a bunch of members I loose a bunch of the original members.
For example
Member number 473 is assigned to line number 1073. After the push member 473 is assigned to line number 357, which is a new line of the new bunch of members, and the cross-section is changed. Meaning that I have lost the original member.

#### Expected behaviour:
I expect that the adapter pushes the new members and assigns it member numbers that are not currently being occupied already by the model.

I believe it should not matter if the existing RFEM list is not a continuous series.

#### Test file(s):
to big, but happy to help test a PR