There is NO WAY to modify that shower. Sorry.

Your options are
1) Live with it
2) rip it out and put in a barrior free shower, google turned up this
http://www.florestone.com/section.php?s=special as the first hit from
http://www.google.com/search?hl=en&l...ee+Shower% 22
3) Have a custom shower built. This may involve a base similar to those
shown in the first link in option 2 but with tile walls or a completely
custom shower enclosure.

Obviously option 1 is the cheapest, probably number 2 is second
cheapest.