Channel Form Multiple Relationship field append and overwrite

Hi

I have a channel with a multiple relation ship field

So for example i might have say 5 entries assigned to the channel field out of a possible 50

I then have a channel form on my front end which allows me to update the entry with the rel field

So when the form is submitted i want to entry_id 700 to be added to the relationship field list

So now i would have 6 entries assigned after the form is submitted

My problem is that when the form is submitted it clears the previous 5 assigned entries and i only have the new entry assigned

is there away that when the form is submitted i can append the entry id to the previous 5 entries?

Thanks

{exp:channel:form channel="cpd" return="members/cpd/success/ENTRY_ID" logged_out_member_id="3" entry_id="{entry_id}”}

<input name=”cpd_members[data][]” type=”text” value=”700”>

<button type=”submit” class=”buttonlarge”>Submit</button>

{/exp:channel:form}