This is actually a limit of the underlying NRefactory engine that powers the Code Converter. I suggest you request this support on the SharpDevelop forums: http://community.sharpdevelop.net/tags/nrefactory/default.aspx
. You can further test the NRefactory engine using the simple converter provided by the core team: http://codeconverter.sharpdevelop.net/SnippetConverter.aspx
We actually do a little "enhancing" of the NRefactory conversions to look for special cases like this (changing parenthesis to brackets). We catch it is some cases, such as Session("key"), but our support is not exhaustive. We'll try to cover more scenarios while we wait on full support in the engine.
Hope that helps.