You might want to post this question to the Pick and multivalue database forum https://groups.google.com/forum/#!forum/mvdbms.
comp.databases.pick has been subject to a lot abusive posts and I wouldn't recommend it. There has recently been a similar discussion
https://groups.google.com/forum/#!topic/mvdbms/aCYFg8NJwFI
There certainly won't be a single button to push export the entire D3 database into CSV files. The D3 data needs to be put into a format that will load into a SQL (probably) type database. The big mismatch is how nested rows of data are handled. Consider an invoice with has fields like invoice number, customer number and data that are simple single valued fields. Then there's the products, quantities and prices. There are multiple rows of these per invoice. In D3 the products quantities and prices are a simple list of values ie the products field is list of product codes. In a SQL type database each row becomes a entry in a table with its own key like invoicenumberdetailline. That sub-table key needs be created either by the process that exports the D3 files or by the process that imports the CSV files into a SQL database. I've seen Pick styles databases with files that have 6 or more different multivalue groups in the same file, that would be 1 D3 file, 7 or more CSV files/SQL tables.
This isnt a difficult process, its just needs some care and time to work through the all the files in your system. If your system has good up to data dictionaries describing your files the process of conversion will be much easier than if your system has no dictionaries and you have to inspect the programs to figure out the file layouts. If the programs a poorly written with terse ambiguous variables names, it could take a long time. I say this because Pick was an easy system to learn, a lot of self-taught-written systems have been created that are less than obvious for other programmers to understand rapidly.
You're really not going to know what your in for until you get a Pickie to look at your system.
I don't know anything about odoo, when I've been involved in data migration we knew what the new system needed. It sounds like you want to export to the data first, then cobble together some open source stuff to use what you end up with?