Daisy chaining switching supplies
In general there are no issues in connecting the output of one kind of power supply to another as long as the load specifications of the source supply are not exceeded.  Adding a boost supply will potentially increase the noise on the source supply, so you should make sure that the input to the boost supply is well bypassed.  

Since your current requirements are so low, you might consider using  a switched capacitor boost regulator, since it can potentially be lower noise than an inductor based boost regulator.