This is a great idea and also possible if you create your a custom Wrapping Forms Container Block.

Have a look at this commit whch adds the files for a custom Forms container: Custom DataCaptureForm Container block with razor template · johnnymullaney/Foundation@2fdb719 (github.com)

1. In the Forms Container Block definition add a ContentReference "Base Form" property which Allows Type Form
2. In the Controller you could implement the logic to merge both IForm.ISteps 

What I've attempted so far is the create a new form, update its ElementsArea with the elements from the new form, and updated its steps with the elements from the new form as well. The added fields render on the page, but on submission, the submission data does not contain the new items. Any advice?

        public override ActionResult Index(ILeadCallToActionButtonBlock currentBlock)
        {
            
            var baseForm = _contentLoader.Get<ExtendedFormContainerBlock>(currentBlock.LeadBaseForm).CreateWritableClone() as ExtendedFormContainerBlock;
            var addOnForm = _contentLoader.Get<ExtendedFormContainerBlock>(currentBlock.LeadFormSpecificElements?.Items[0]?.ContentLink);
            foreach (var addOnItem in addOnForm.ElementsArea.Items)
            {
                baseForm.ElementsArea.Items.Add(new ContentAreaItem()
                    { ContentLink = addOnItem.ContentLink });
            }


            var baseFormLastStep = baseForm.Form.Steps.Last();
            var baseFormLastStepElements = baseFormLastStep.Elements;
            var addOnFormElements = addOnForm.Form.Steps.First().Elements.ToList();
            foreach (var addOnFormElement in addOnFormElements)
            {
                addOnFormElement.Form = baseForm.Form;
            }

            var combinedElements = baseFormLastStepElements?.Concat(addOnFormElements);

            var newForm = baseForm.Form.Steps;
            newForm.Last().Elements = combinedElements;

            baseFormLastStep.Elements = newForm;

            var viewModel = new LeadCallToActionButtonViewModel
            {
                LeadForm = baseForm,
                ButtonColor = currentBlock.Color,
                ButtonText = currentBlock.ButtonText,
                TopText = currentBlock.TopText,
                Image = currentBlock.Image,
                Anchor = currentBlock.Anchor,
                Id = new Guid()
            };
            base.Index(currentBlock);
            return PartialView(viewModel);
        }