You can create a checkbox group field based on the "load from channel field" option, which will create a list of checkboxes with either entry titles or content of a custom field as labels.
If you're looking for more flexibility, however, the only option available is to manually create your own checkbox group list in a template. This can be done by using the {exp:freeform:form} tag and creating your checkbox group using {exp:channel:entries}, {exp:channel:categories} or whatever tag you need to create the list.
For example, let's say you created a field (textarea) called form_events. You would create your dropdown like this:
{exp:freeform:form /* Your other parameters here */}
<h3>Choose your events</h3>
{exp:channel:entries channel="events" orderby="date" dynamic="no"}
<input type="checkbox" name="form_events[]" value="{entry_id}" />{title} - {entry_date format="%Y-%m-%d"}<br />
{/exp:channel:entries}
// The rest of the form
{/exp:freeform:form}
This way you have control over the checkbox labels, the ordering, etc.
The above field example will save your selected "event" channel entries as newline-delimited entry_ids in the form_events Freeform custom field. Please note that the brackets [] in name="form_events[]" are necessary in HTML forms to submit and save an array of form data.
