Yes, CBSubs has its own substitutions provided by its Joomla system plugin. You can use them anywhere Joomla content prepare plugins are processed. So if you need this in a Custom HTML field be sure you enable content plugins under the parameters tab.
The below article covers some of the usage.
www.joomlapolis.com/news/18550-cbsubs-gpl-4-1-with-timed-content-taxes-inclusive-price-display-and-plan-substitutions-usable-anywhere
Review the description of the Joomla system plugin "cbpaidsubsbot" within Extensions > Plugins for further usage information.
Based off your example you'd have the below.
Code:
[cbsubs:if plan="1"]I am subscribed to Plan1[/cbsubs:if]
Note where 1 is it's your plan id. You can find your plans ids within CBSubs > Plans in the far right Id column.