You are legally entitled to be treated like a regular employee by the recipient employer for retirement plan purposes if you are a common law employee of the recipient employer, regardless of any pension plan of the leasing organization. A common law employee is defined as a worker who performs services for an employer who has the right to control the result of the work and the way in which it is done.
If you are not a common law employee, but have worked for the recipient employer on a full-time basis for at least one year, you must also be treated as a regular employee for retirement plan purposes. However, in that case, the recipient employer does not have to cover you under its plan if you are covered by a suitable plan through the leasing organization.
Please note that you are still subject to any requirements that regular employees must meet, such as job tenure and minimum hours requirements.