I am looking for the possibility to have personalized subject line in mail merge function.

Suppose the data source have a column called subject and I would like to be able to reference this field in the send mail merge dialog, e.g. <subject>. Or generally speaking reference combined fields, e.g.: foo <field1> bar <field2>.

It seems that there is another extension available to provide this feature, but not for LibreOffice: [1].

Another option would be to support this feature in core mail merge feature of LibreOffice.

[1] https://www.mapilab.com/blog/how-to-customize-subject-line-in-mail-merge-toolkit-for-outlook/