Tying a switched line into a switched line
Electrical work should really be done by a licensed Electrical Contractor.
Please only undertake DIY wiring if you REALLY understand what you are doing.

What you are suggesting is not ideal, and should be avoided if possible.
How are you planning to switch the new fittings?
Sometimes, for example when retrofitting say an exhaust fan to a bathroom or WC, there is no access, and the only way is to tap into the light, generally using a pull-cord switch.  Not ideal but sometimes unavoidable.