Yes - we've been told that in the case of an absent parent it would be possible but are struggling to find anything concrete to guide us through it. We are at the very early stages of looking into it all.
If you haven't been there already, there is an Adoption board under 'Becoming a Parent' and there may be people there who can guide you.
Broadly speaking you will need to contact your local authority social services. The absentee parent will need to be very absent, otherwise they will need to give consent. If the child is old enough their views will be taken into account. Adoption gives the adopting step parent full legal rights and responsibilities. Including if the relationship breaks up, just as if they were a birth parent.