Without seeing your code it is difficult to give you an exact answer. There is the shortcode_atts_{$shortcode} filter that was introduced in WordPress 3.6. All attributes is run through this filter. It has to be noted that the $shortcode parameter needs to be set in shortcode_atts. I haven’t seen shortcodes yet that has this parameter set though.
Your problem here is actually bigger that just the attributes as you will need to add the attribute and the post_type parameter to your query arguments. If the author did not supply a custom filter for the query arguments, then you are at a dead end street.
IMO, you have two choices here
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Contact the theme author and ask him to at the relevant code to the shortcode in his next update. I don’t think that he/she will have an issue as this adds value to the shortcode
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Create a child theme and copy the shortcode to the child theme. Modify the shortcode as needed, and remember to change the function name and shortcode name accordingly. This way you will have a shortcode that fits your needs