I assume you want to connect the top of the missile tower and the botton of a siege tank, visually? If this is correct, it is pretty much impossible to do without editing the models.
You can try to use some available models to get what you need; the pack should include the bottom half of a siege tank, not sure about the missile tower.
Also, you probably want to consult an attachment tutorial.
This is the wrong section, by the way. This section is for melee map development only, your question probably belongs in the data forums. A moderator can move this post for you.