I am making a chocolate + oat stout on the weekend and would like to add some additions (cacao nibs predominantly - toying with adding some rum soaked raisins). I do not however have a suitable secondary fermenter. Will adding these to the primary cause any issues and is there anything else I should consider?
If you don't have a secondary then, feel free to add them to your primary. You don't really have to worry much about making additions in your primary, I have done it many times in the past when I lacked a spare FV to use as secondary, and suffered no ill effects.
You may just have to add a little more of any flavourings you are adding as some of the flavour compounds may stick to the yeast and drop out with the yeast adhered to the surface of the cells.
I agree with previous answers: if you have no secondary, there's nothing wrong with chucking whatever you want to add into the primary. However, if your additions are highly aromatic, you may want to add them only after 3 days or so when the fermentation begins to calm down a little, or the vigorous bubbling of the initial fermentation stage may carry off most of your aromas.