Dear,
if the noise sources in the splitter are thermal noise sources then spliiter NF is equal to splitter loss, 1\L (L<1), (at T=T0).
The noise power at the splitter input port is k*T0*B (thermal noise): this is attenuated (by the losses) to K*T0*B*L at the output port, but the splitter adds itself k*T0*B*(1-L) at the output port. The total output noise power is K*T0*B. Calculating the NF, as (Si/Ni)/(So/No), you obtain NF=1/L (>1) !
This is true only for thermal noise sources; if the noise sources are not of thermal type the output noise power added by the network is k*T*B*(1-L), where T may be greater or less than that of the network itself.
Bye