Not without writing code to implement that. The activity form can be extended with the following triggers.
New Post
Code:
$_PLUGINS->trigger( 'activity_onDisplayActivityStreamNew', [ $rowStream, &$row, &$form ] )
Edit Post
Code:
$_PLUGINS->trigger( 'activity_onDisplayActivityStreamEdit', [ $rowStream, &$row, &$form ] )
There's 2 options for this trigger. Either manually append to $form or just return your custom input and it'll be inserted automatically. This can be done using CB Auto Actions. For CB Auto Actions var1 is the activity stream object, var2 is the activity object itself, and var3 is just the form array.
Now that display is handled you'll need to save the data. The below triggers handle the save process.
Before Edit Post
Code:
$_PLUGINS->trigger( 'activity_onBeforeUpdateStreamActivity', [ $rowStream, $source, &$row, $old ] );
After Edit Post
Code:
$_PLUGINS->trigger( 'activity_onAfterUpdateStreamActivity', [ $rowStream, $source, $row, $old ] );
Before New Post
Code:
$_PLUGINS->trigger( 'activity_onBeforeCreateStreamActivity', [ $rowStream, $source, &$row ] );
After New Post
Code:
$_PLUGINS->trigger( 'activity_onAfterCreateStreamActivity', [ $rowStream, $source, $row ] );
For CB Auto Actions var1 is again the activity stream object, var2 is the source object for the post (e.g. if this is a groupjive group post this would be the group object), var3 is the activity object itself. The easiest way to store custom data is to just push it into params storage for the activity post, but you can implement whatever storage you want. Example of pushing data to storage.
Code:
$row->getParams()->set( 'my_custom_field', 'value_for_my_field' );
As for displaying custom content it can either be done using template files or using the below trigger.
Code:
$_PLUGINS->trigger( 'activity_onDisplayActivity', [ &$stream, &$row, &$layout[
'title' => &$title,
'caret' => &$caret,
'date' => &$date,
'message' => &$message,
'content' => &$content,
'media' => &$media,
'header' => &$header,
'footer' => &$footer,
'menu' => &$menu,
'compact' => &$compact,
'isDetails' => false,
'isNew' => $isNew,
'isRow' => $isRow,
'isModal' => $isModal,
'isInline' => $isInline,
'isSimple' => $isSimple,
'isEmbed' => $isEmbed,
] ]
The $layout array gives access to multiple positions to insert content. Alternatively just returning content on usage of this trigger will insert content into a post.
A backend UI to add new form elements won't be implemented until sometime after CB 3.x development is complete as we have to generalize our field management (we already have plans for doing this) so fields can be used anywhere instead of just profiles.
I can provide additional instructions or even a simple example using the above if needed, but beyond that you'll need the coding experience to implement anything further.