...to Trados Support - please use this forum to post Trados-related topics.
The translationzone site does not give any information, if such an upgrade is possible and if so, how much it would cost - assuming they would let me upgrade from a (no longer usable) dongle version to a softkey version.
Shouldn't be a problem, assuming you can still provide your license number.
Details are shown on TranslationZone - click on Upgrade from pre-TRADOS 6 FL.
What really concerns me though is, whether I would be able to use my existing 5.5 TM without problems in 7.0?
Yes. It's recommended to export your version 5.x TM, and to re-create it under T7, though.