Adding costs to an existing (in progress) project

Hi,

This point is takien into account in ProjeQtOr and it should be very easy to set-up.

1) I would like to add resource costs to a project that is already in progress: what is the best way to do this ?

Just add resource cost in the resource screen, for all the used functions of this resource. Actrivities and project cost will be updated automatically

2) For an existing assignment, the cost is greyed out, so cannot be modified. I thought of using the "import data" functionality to update the cost of assignments, but I see two fields : dailyCost and NewDailyCost. What is the meaning of these two fields ? Can I modify one of these when importing a CSV file ?

Yes, cost is alway retrieved from resource function cost. It cannot (and shouldn't) be changed manually. Do not use import for this.
The assignment cost is mostly informative : it is the initial cost of the resource.
If you wish to change it, you'll have to switch each assignment to a new function, and then set it back to correct one : this will update assignment cost.
But keep in mind that this cost is not used for real and left costs calculation.

3) Finally, I am also a bit confused about the "start date" for a resource's cost: Since the "start date" for the first cost is greyed out, what date is taken into account ? I'm wondering because some existing activities show non-null real cost, while the corresponding assignment shows a cost of 0 (existing actiivty with real work in the past, and today I added a cost of 400€ to the resource). See screenshot for a better explanation.

You don't have to define start date when you have one only cost : it is always valid
Start date is only used when cost changes. Then start date indicated when new cost has to be taken into account. And previous cost automatically get an end date just the day before.
Most important : if you define a start date in the past, and that real work already exist after this date, real cost will be automatically updated with good value
Last important point : left cost is always calculated from latest function cost, whatever the date of the new function cost.