Bold Subscriptions and Bold Upsell are compatible as long as your Subscriptions style is set to "Multiple Products" or "Recurring Cart". This allows you to trigger upsell or cross-sell offers when someone adds a subscription to their cart, or goes to checkout from the site.
Upsell is not able to offer subscription items as upsell/cross-sell products. The subscription information does not get passed to the cart correctly when they are used as the offered item.
It doesn't work with "Single Product" style because the customer never actually enters the Shopify cart, which is where the upsell is offered. With "Single Product" style, the customer is taken directly to the subscription checkout.
Visit Use Bold Subscriptions in Recurring Cart Style for setup instructions.