I'm currently working on a campaign for my DnD group. I want to create a monster that is nothing like anything in the MM. I want to add a couple abilities that some monsters have but are not on the features list. The abilities i want to add are Paralyzing touch from the lich and luring song from the harpy, as i said these features are not on the list so i dont know what to add for the CR. I'm sure the they have to add something i just dont know what.
It is very possible that i have missed something somewhere, it wouldnt be the first and im sure not the last.
any actual help will be apperciated. flames wont be 
The effects of those sort of abilities are not really easy to model with a change in CR.
On the one hand, you could model them by using the effective increase in danger of the conditions they apply, such as treating paralysis as +5 to attack rolls and the increase to average damage per round that getting critical hits would give.
On the other hand, there are probably solid chances that the abilities never actually come into play because they are successfully saved against or otherwise removed before they can be exploited.
My opinion on them is this: they aren't on the chart in the DMG because they don't have an overall effect on CR - their bonuses are considered to be counter-balanced by their difficulty to exploit.
I'd say go for it, especially if some of your players have their own copies of the MM. As for the CR, I'd just eyeball it based on a comparable monster, maybe increasing it by one or two depending.