bbcode-to-md happily ignores all phpBB's lists. The list syntax is
[list][*]item 1
[*]item 2
[/list]
and
[list=1][*]item 1
[*]item 2
[/list]
respectively for [ul] and [ol]s. Luckily, phpBB adds closing tags for [*] items. My workaround simply converts phpBBs lists into bbcode using [ul] and [ol] which then can be converted by the standard bbcode-to-md code.