ALA must be converted to EPA and DHA, which it does at less than 10% efficiency, so you'd need to consume an assload of it, too. Your best bet is still fish(oil).
and you can actually use up all the various desaturase and elongase enzymes in the process so that 10% upper bound drops markedly as you consume more ALA. Put differently long chain n-3's production definitely isn't linearly dependent on short chain n-3s like ALA and stearidonic acid.