Hi,
Yes it is possible to train TAPAS on other custom aggregations. You can change the number of aggregation operators in TapasConfig, like so:
from transformers import TapasConfig

config = TapasConfig(num_aggregation_heads=10)

and then initialize a TapasForQuestionAnswering model with a pre-trained base and your custom head on top:
from transformers import TapasForQuestionAnswering

model = TapasForQuestionAnswering.from_pretrained('google/tapas-base', config=config)

For more information, see the fine-tuning guide of TAPAS here 1.