Spectrum Module
Using Jamroom
It certainly is possible. A module made by The Jamroom Network is no different than a module Made By You. Even the core of jamroom is a moudle.
So modules are a core concept in jamroom.
You can create either a dedicated module that does something specific or a general module where you chuck all your changes for your specific site.
From there..... actually...
What might be more useful to you is: there already is jrCore_page_custom().
Eg:
$html = '<div id="pl-addon-options"><!-- addons load here --></div>';
jrCore_page_custom($html);
It will depend a lot on whether you're wanting to use the Form Designer to build the form or if you're building the form in php. The code above allows you to pass anything you want in and make it part of the form.
You'd generally use that method when its your modules form. If you're injecting into another modules form then either the Form Designer method or the Events and Listeners system are other ways to accomplish changing whats in a form.
For the form designer method you'd make a custom form field type that could be added via the Form Designer. To do that you'd use your module to create a form field type.
Lets look at 'daterange' since its very specific and is easily searched on in your IDE and is pretty simple.
First the jrCore module adds this line to its _init() function to register the form type:
jrCore_register_module_feature('jrCore', 'form_field', 'jrCore', 'daterange');
Then these 3 functions are defined (you could do this in include.php, but the core is a big module so for tidyness it has an included form.php file where all form stuff is)
function jrCore_form_field_daterange_display($_field, $_att = null)....
jrCore_form_field_daterange_form_designer_options()
function jrCore_form_field_daterange_attributes()
function jrCore_form_field_daterange_assembly($_field, $_post)
If your custom field type was really simple you might not need _attributes() and _assembly().
Take a look at those functions, see if that's enough to get you going.