Howto make my own gmapsupp.img for my old Etrex Vista Hcx ?

[HWe]

I own a Garmin Etrex Vista Hcx. The device has a track limit of 20 track with max. 500 points each --> 10kpoints maximum
The tracks are stored in the internal memory and not on the SD-card.
the SD-card (2GB) can only store the map-image (gmapsupp.img) and the trace-logs.

I want to store a big GPX-track (>10kpoints) on the device. Since this is not possible via the "tracks", my idea is to
1. Use an existing map-image (gmapsupp.img or any other source)
2. add the GPX-track to the map in a certain colors (e.g. pink or light green) on a certain layer to clearly see it on the map - just like a street or river
3. save the updated map back on the SD-card (gmapsupp.img)

With this i would have the big GPX-track as a part of the background map in my Vista Hcx.
The goal is - while hiking a long-distant trail - to verify that I am on-track.

I did not find any tutorial how to create custom map-images (gmapsupp.img) for such a use-case.
Any idea ?

 

[Nuvi-Nebie]

It is possible to edit your original 10K+ Track using a free tool such as Garmin Basecamp, the Track editor has a filter option that can reduce the number of track points, in this example a 9317 point Track is reduced to 480 points, if the resulting Track is not acceptable due to the lack of points you can split the original Track using Basecamp into multiple tracks and then reduce each section to <500 points so the multiple tracks can be used on your Etrex Visa