the two plugins should use a similar structure though it could be some of the rule formats arent exactly the same. if thats the problem then you'd have to make an import filter. one easy thing though, check $rcmail_config['managesieve_script_name']
and $rcmail_config['sieverules_ruleset_name']
in the 2 config files are set to the same thing, the value they had in your managesieve setup.