Well, I made some progress.
I found in these forums the City Navigator Israel 2011.40 map - the version which does not need special hebrew fonts.
It can seem to work on the Nuvi 750's SDHC card if it is the only map on the card.
But two files named gmapsupp.img(NA and EU) and gmapsup2.img(IL) in the SDHC card's "F:\garmin\" directory did not seem to work. Only the first map would be recognized. This makes me suspect FID conflicts? Some conflict between EU and IL maps?
So, one approach is to use two SDHC cards, one for NA/EU and the other for IL.
In addition, if I used GmapTool to convert the Israel img file into files for MapSource (and run the install.bat file), then Mapsource shows the Israeli map in the list of maps, but no map is visible.
I then used GmapTool to combine the two img files into one img file.
After booting up the nuvi750 with the SDHC card, it appeared to have all maps available in the settings,
but when specifying where to go - setting the country to Israel, it wants a state - which is not applicable for Israel.
What does work?
Disable all maps but Israel map, and it seems to work - at least I can specify an Israeli city and street and see it on the map.
Then I can disable the Israel map and enable the NA and EU maps - and they seem to work ok, as I can look at Rome, Italy and also places in the USA.
So, I consider it sort of a success - with one img file containing NA, EU, and IL which is just under 4GB.
It appears the Israel map system is so different that to use the Israel map requires disabling all other maps in the nuvi's settings.
This is not bad for a refurbished Nuvi 750 , case, and bean bag mount - alltogether cost me under $100.