Can I work on a freelance basis as well as a salaried employee?

In short: you certainly can! There are, however, a few things to bear in mind.

Freelance work is directly separate from salaried work. So you decide how much you work as an employee and how much you work on a freelance basis. It is important, however, to communicate this to your employer. 

In addition, the tax authorities do add up both gross incomes. When you do your income tax, it is therefore possible that you will end up in a higher tax bracket. This means that you have to pay a higher percentage of tax. However, there is one exception where this is not the case. Namely when you are not seen as an entrepreneur by the tax authorities. If this is not the case, you do not have to declare your earnings from your sole proprietorship. Are you seen as an entrepreneur and do you want to know in which tax bracket you fall? Check the following link on the website of the tax authorities.