The party's shadow Culture spokesman, Don Foster, is to meet the NCVO this week to discuss how the principle should be defined, and is expected to table an amendment to the Bill within the next few days.
The concept of additionality - how lottery funding should differ from Government spending - has been a source of controversy since the Lottery was created, and the debate flared up again recently over the Big Lottery Fund's support for the Government's School Food Trust.
The Government has said that it still supports the additionality principle but has refused to spell out what it means in the Bill.
The NCVO's preferred definition states: "Lottery funding should be additional to what should properly be funded from general taxation and not a substitute for it.
It should not be used to fund essential services or government-inspired programmes."
But parliamentary officer Pete Moorey said that the definition needed more work and would be hotly debated by MPs.
Sport and Tourism Minister Richard Caborn gave an indication that the Government might accept an amendment when he told the Commons that "it was accepted that additionality was an important principle that should be embodied in future legislation".
But he added that the principle had "prevented the lottery from being as holistic in its approach as it should have been".
The Lib Dem's Foster has also called on eBay to introduce a charity auction site as a way of making amends for initially allowing individuals to sell their Live 8 tickets on the site.