The Swiss authorities online
If you want to adopt your spouse’s child, you need to meet the following basic requirements:
The biological father / biological mother is generally required to give his/her consent to the adoption.
However, if the parent is unknown, has been absent for at least two years with no known address, is of impaired judgment or shows little or no interest in the child, this requirement may be waived.
You need to contact your cantonal authority responsible for matters relating to adoption.