I havent seen a standard tool to migrate userdata into UCS until now. The main reason is most likely that migration projects will have different preconditions about the data to migrate.
There is a CSV-import module for UCS@School but in this case the focus is more limited as in the standard product.
The standard way to create users during a migration is the udm CLI tool.
DOC: [ur=http://docs.software-univention.de/manual-4.0.html#central:udml]4.9. Command line interface of domain management (Univention Directory Manager)[/url].
The usual procedure is to write a small script which is parsing the sourcefile containing the data and calling "udm users/user create ..." for each object.
Source data can be a text-file like CSV or LDIF, Apple Open Directory is a fork of OpenLDAP and should allow creation of LDIF as well.
There are some useful hints in Migrate Existing Samba 3 Installations to UCS 3 with Samba 3. This article has a link to a GitHub repository containing a script collection which might give an idea what can be done, even if the conditions are different.