When I click on "import data", the usual window is displayed with all the markers with [ext] in front of them. If I click in any of them, the option where we can specify the time window for our epochs becomes unavailable.
Your events are read as extended events:
https://neuroimage.usc.edu/brainstorm/Tutorials/EventMarkers#Extended_events
These events already have their own duration defined, you can't redefine that when importing these time segments:
https://neuroimage.usc.edu/brainstorm/Tutorials/Epoching#Import_in_database
If you want to enable this option, you need to convert the extended events to simple events first:
https://neuroimage.usc.edu/brainstorm/Tutorials/EventMarkers#Other_menus