According to Mahayana, a person can become a buddha after reaching nirvana. We hold that all sentient beings will, sooner or later, become buddhas - be it after they reached nirvana or not.
As to the hearers and solitary realizers, we say that while they dwell in abiding nirvana, the buddhas manifest to them and encourage them to enter the Mahayana path, so that they will in turn become buddha - thereby achieving their own benefits completely, and the benefits of others. This is the presentation of the Abhisamaya-Alamkara:
The arhat without the contaminated aggregates will hear the voice of
the Buddha telling him or her something like “you are free from the
suffering of samsara, but still you have not gained nonabiding
nirvana. Therefore, you need to enter the Mahayana path.” With these
words the Buddha wakes up the arhat from their blissful meditative
In this context, there are two types of nirvana:
- Abiding nirvana. It is the nirvana of hearers and solitary realizers. We refer to it, in a somehow derogatory fashion, as "individual liberation".
- Non-abiding nirvana. It is the final true cessation, the nirvana that bodhisattvas attain when they achieve full perfect enlightenment - the path of no more learning.
In his commentary to Bodhisattvacharyāvatāra Geshe Gyaltsen said:
Non-abiding, the nirvana of a buddha, is a nirvana not abiding in the
two extremes. It is not the nirvana of a hearer or solitary realizer,
but the nirvana of the highest state of development of a buddha.
The following passage also shows that the one who has attained abiding nirvana did not abandon all objects of abandonment.
Abiding nirvana is a temporal goal. Temporal has the connotation of
being ordinary. This is because there is still something to abandon.
But it is tricky because the term 'nirvana' is also synonymous with
'[final] true cessation'. And the true cessation of a buddha is not
something that is ordinary.
There are buddhas who did not enter abiding nirvana, but buddhahood is non-abiding nirvana. After a bodhisattva enters non-abiding nirvana, he abides in neither of the two extremes of peace (abiding nirvana) or samsara. This is because here, samsara is understood as the contaminated aggregates. A buddha is free from contaminated aggregates, and he can manifest countless enjoyment body and emanation bodies. For instance, we say that an already enlightened one showed the aspect of being born as Shakyamuni, and then showed the aspect of undergoing austerities and achieving enlightenment while he was, in fact, already enlightened. We say that Shakyamuni buddha was a supreme emanation body, one of the countless bodies a buddha can emanate so as to benefit sentient beings.