This is not a hindi word, but a name so depending on how you pronounce it the transliteration changes. do you read double L distinctively ? Also, is the last N nasal? both L wrtiten as one and last N not nasal = अलिसन/ अलिसोन both L as one and N nasal -= अलिसं/ अलिसों both L distinctive and N nasal = अल्लिसों both L distinctive and N not nasal = अल्लिसोन so on and so forth :)

Allison would be written as ऐलिसन. I think. Because अ is pronounced as uh and ऐ is pronounced as a. Hope that helps =)