Is the two-rak'ah prayer after ablution obligatory for one who is not accustomed to performing them, with the intention of entering into a state of ihram thereafter?

There is no harm in praying two rak'ahs and then entering into ihram after them, even if these two rak'ahs are not a regular practice of the worshiper. As long as the sunnah of ablution (wudu') is legislated and one intends to pray, there is no objection to it.