importing myisam 5.0 database into a 5.5 innodb server
A mysqldump will attach ENGINE=MyISAM and ENGINE=InnoDB to every CREATE TABLE statement.

You should be fine. 

WARNING : There is one gotcha !!!

The mysql database itself (where the grant tables reside) have different column layouts between versions of mysql.

You can dump all databases from MySQL 5.0 EXCEPT the mysql database.

It is much better to port the mysql grants using pt-show-grants.

Here is my personal emulation of what pt-show-grants does

mysql -hhostaddr -umyuserid -pmypassword --skip-column-names -A -e"SELECT CONCAT('SHOW GRANTS FOR ''',user,'''@''',host,''';') FROM mysql.user WHERE user&lt;&gt;''" | mysql -hhostaddr -umyuserid -pmypassword --skip-column-names -A | sed 's/$/;/g' &gt; MySQLUserGrants.sql