Embed RSForm in hikashop checkout registration

-- HikaShop version -- : 2.5.0
-- Joomla version -- : 3.3.6

Hi
I have an RSForm registration form , through which all the registrations in the website take place.
I would like to embed this form in the Hikashop Checkout process (rather than the default joomla registration form)
is that possible?

What are the reasons why you want to use a form component rather than the built in custom fields capability in HikaShop? Does the form integrate with an external system or reporting in some way?

If you can use the native Hikashop custom field features you will save yourself a lot of customization work and hassles at upgrade time.

I want to have a customized registration experience , Hikashop custom fields will only help during a checkout process
while RSForms Registration form can be used even outside the context of a shopping use-case

actually the Hikashop registration form doesn't fulfill my requirements, I want to keep using both RSForms and Hikashop
but I want to sync the user stores of (joomla core, Hikashop users and RSForms users)
I wonder what is the best way to incorporate the checkout page in this synchronization, please help me with these choices and the way to accomplish them
1- load the RSForm inside the checkout page, by possibly updating some rendering php function that indstead of displaying specific fields, will fetch a RSForm
2- keep the Hikashop registration form , but on the register event (some do_register function in a controller) I will have to implement a database insert that inserts the supplied data to RSForms userstore (in addition to the original code of course)
can you please help me by telling me which one is easier, and what are the files to look at

yes I am aware of that, I just thought that maybe the form it self is being rendered by a php function, this way I can hook another php function
two more question please
1-the css of the checkout page, where can I find it ?
I think that the text boxes of the registration are a little large( in hight ) I want to make them smaller
2- when I activate the Hikashop as a registration form of the joomla website, the registration page that appears, is actually displayed under the product listing menu , how can I prevent that ?