Hi droida,
Its possible alrigth, but it looks like a simple workaround with a hidden menu item and a simple module override is needed.
What you could do to get your 'signup' link is to:
1. Create a new "hidden~unpublished" menu module.
Goto Menus->Menu Manager->New and name your menu module something like 'menhidden'.
When saved you can see a new unpublished module in the (Extensions->Module Manager->See menhidden (unpublished)). Just keep it unpublished.
2. Create a new published link in the menhidden menu.
Goto Menus->menhidden->New->Choose 'User'->Click 'Default Registration Layout'
Give it a Title and the important field here is Alias where you type 'signup'
Now you have a published link, in an unpublished menu module.
Can see you use a mod_yoo_login module, and for this you can now create a simple template override to avoid touching the module code and instead use a override.
You will need to copy the file /modules/mod_yoo_login/tmpl/default.php
to a new folder below the html folder in your active template.
Copy the file default.php to: /templates/rt_mixxmag_j15/html/mod_yoo_login/
Open the file in the new location for to edit line 113
Code:
line 112 <span class="registration">
line 113 <a href="<?php echo JRoute::_( 'index.php?option=com_user&task=register' ); ?>"><?php echo JText::_( 'REGISTER'); ?></a>
line 114 </span>
where you replace the 'index.php?option=com_user&task=register' with 'signup'
Save this template override file and try check the link in frontend.