Lawmakers plan to submit a bill to the State Duma on Friday that would make it easier for Russia to absorb foreign territories.
The bill proposes letting parts of other countries join Russia on the basis of a referendum or a decision made by the territory's elected parliament.
On Thursday, the Crimean parliament voted in favor of holding a referendum on May 25 on whether to extend the region's autonomy.
The peninsula, given to Ukraine by Russia in 1954, has a large ethnic Russian populace, many of whom have refused to accept the new government established in Kiev, preferring instead to lean toward Moscow.
"The initiative is ready and will be submitted today," A Just Russia member Mikhail Yemelyanov said, Interfax reported. "Since this is a national issue, all representatives from all factions are free to sign on to the bill."
The bill will amend a law passed in 2001 that stipulates that a "new entity" can join Russia by signing a treaty with the state that it belongs to.
Yemelyanov also said that a bill making it easier for Ukrainians to become Russian citizens will be introduced for the Duma's consideration on Friday.
Under the proposed legislation, Ukrainians would only be required to know Russian language to get a Russian passport.
Yemelyanov added that A Just Russia considers it necessary to preserve the territorial integrity of Ukraine.