I know you can create a Titanium Backup from a Nandroid Backup using the Titanium Backup PRO app.
But is there a possibility to create a Titanium Backup from a Nandroid without having a phone, i.e. using a pc (preferably Linux, Windows is ok too)?
Titanium Backup is able to read from Nandroid backups, so you can use it to restore parts from them -- and then use Titanium Backup again to backup those restored parts. However, as there's no Titanium Backup for PC, this has to be done on an Android device.
If your goal however is to separate some apps from a Nandroid backup, there are other ways to achieve this: Nandroid copies the entire partition (see: nandroid tag-wiki). So you could mount the images (on Linux, this should be no problem with the EXT file systems at least -- see e.g. Restore only part of nandroid image), or unpack them (for YAFFS partitions e.g. using
unyaffs -- see my answer here).
For completeness: there are also ADB-style backups (which again Titanium Backup is able to access). For these, you might want to take a look at e.g. Android Backup Extractor and Android Backup Splitter, and also my answer here on how to convert an ADB backup into a tar file to investigate, using the Perl AdbBackupRoutines from this XDA thread.