4000+ persons converted to Hinduism . see https://www.hindujagruti.org/news/2100.html
Where you can legally convert to Hinduism? Goto any Arya samaj temple. After conversion, you will be provided legally valid certificate.
Many Hindu/Vedic sects will convert you, such as Arya samaj Shaiva Siddhanta Church , BAPS, ISCKON etc. Some sects will not convert one. Those sects who will not convert the desiring person, let them choose not to do so, but they can not speak on behalf of all the sects.
If in case you are asking that whether old religious texts mention about conversion .
Argument1) There was no explicit law in vedas or scriptures, as there was no threat. But the basic principle of not a stuck dharma, but a living dharma, which can fight new challenges was already put by Krishna, which can work for Conversion
"Hinduism has been in existence for thousands of years, however it is only after the advent of Christianity and islam that conversion has become a threat to the survival of the religion. During the ancient times there was no such threat, hence the Rishis did not formulate any rule as such for reconversion. However any threat against Sanatana Dharma was met head on. That is why our Gods and Godesses are armed. They have not preached ahimsa of the mahatma variety, instead Lord Krishna terms any refusal to fight against threats to Dharma as cowardice and sin (kshudram Hrudaya dourbalyam). Refer Bhagavad Gita chapter 2, sloka 3, & 31 to 35. Now there is a serious existential threat against our religion and hence we need to put in place the necessary defenses. Making reconversion easy should be one of them." ref: http://www.haindavakeralam.com/how-to-convert-to-hinduism-a-request-hk4116#comment-158945
Argument 2) you can see lots of people from middle ages converting from other religions to hinduism here.
- https://en.wikipedia.org/wiki/List_of_converts_to_Hinduism
- https://en.wikipedia.org/wiki/Category:Converts_to_Hinduism
This is going from centuries. Even before the most orthodox middle ages. Esp, Mihirkula like Huna emperor (Mihirakula ruled his empire from 502 to 530CE ), who destroyed buddhist, hindu sites,killed monks. Later converted from his barbaric native religion to Hinduism.
So obviously during 500 CE also, we have documented cases of conversion. So, obviously conversion was allowed. Only thing is that during dark middle ages, when all the world became too much rigid, then our caste system and conversion also changed to inflexible. And Many people still carry the same impression on conversion which was during dark middle ages, in between. But our ancient india was not so inflexible. When new invadors came and tried to assimilate later here in the culture, they were welcomed then. Though nobody tried to evangelize actively to others, like buddhists.