Eigther study the B3D Format and learn how to nest NODES to save multiple Child-Meshes inside one File, or, maybe easier, use the AddMEsh Command to add multiple MEshes together before you save it.
x1=loadmesh("chair.x")
x2=loadmesh("table.x")
x3=createmesh()
positionmesh x2,213,34,anywhere
addmesh x1,x3
addmesh x2,x3
freeentity x1
freeentity x2
now start with the saveB3D code to save x3
x1=loadmesh("chair.x")
x2=loadmesh("table.x")
x3=createmesh()
positionmesh x2,213,34,anywhere
addmesh x1,x3
addmesh x2,x3
freeentity x1
freeentity x2
now start with the saveB3D code to save x3